I'm pretty sure that AlertMe uses some form of encryption between sensors and the Hub. At a guess, you'd need to appear to be an AlertMe Hub before the sensors would be happy to talk to you. I may, of course, be entirely wrong - it might be post-pairing that the security kicks in.

Hmm. Interestingly, though, you can pair AlertMe-brand sensors with the British Gas Safe & Secure system. I may well be talking via my bum.

I think what happens is it logs in, and then expects some sort of register write from the coordinator. If it doesn't get that within some window, it assumes it paired to the wrong thing and moves on to look for other pans.